log4j 1.2's SMTPAppender supports the following adresses:
-from (1)
  <param name="from"   value="log4j@hostname.example.org" />
-to (many)
  <param name="to" value="somebody@example.org,alert@example.org" />
-cc (many)
  <param name="cc" value="one@example.org,two@example.org" />
-bcc (many)
  <param name="bcc" value="three@example.org,four@example.org" />

Sometimes, it's necessary to set the from address to an internal address
that works with the SMTP server for outgoing messages, but when a receiver
replies to the address, it won't because that internal address is unknown
to the receivers SMTP.

The solution for this is to specify one (or more) ReplyTo addresses. Currently,
the SMTP Appender does not support this:
  <param name="replyTo"   value="support@example.org" />
or
  <param name="replyTo"   value="support@example.org,support-alert@example.org" />


The attached changes in src/main/java/org/apache/log4j/net/SMTPAppender.java
would implement this capability.